Apparatus for forming monocrystalline ribbons of silicon

ABSTRACT

An apparatus for pulling monocrystalline ribbons of silicon from a melt of silicon, which includes a shaping guide, through which the ribbon is pulled, having a heat retentive coating of a material, such as platinum, on the top surface of the guide. The shaping guide preferably includes 45* converging beveled lips on the bottom side thereof oriented toward the melt from which the ribbon is drawn for directing the melt into a rectangular slot formed in the shaping guide near the top surface. The rectangular slot serves to form the silicon in shape of a ribbon. The apparatus also preferably includes a heat reflective member supported above the shaping guide by a quartz pedestal. The heat reflective member serves to assist the coating on the top surface of the shaping guide in maintaining a low temperature differential between the top and bottom surfaces of the shaping guide.

United States Patent [72] Inventor James Claude Boatman Richardson, Tex. [2]] Appl. No. 730,793 [22] Filed May 21, 1968 [45] Patented Nov. 2, 1971 [73] Assignee Texas Instruments Incorporated Dallas, Tex.

[54] APPARATUS FOR FORMING MONOCRYSTALLINE RIBBONS 0F SILICON 19 Claims, 3 Drawing Figs.

[52] US. Cl. 23/273 SP, 76/107 AS, 18/8 SS [51] Int. Cl B01j 17/18 [50] Field of Search 23/273,

301; 264/93; 76/107 AS; 18/8 SS [56] References Cited UNITED STATES PATENTS 2,686,212 8/1954 Hornet al 23/301 2,839,783 6/1958 DeWolf 18/8 2,927,008 3/1960 Shockley 23/301 3,251,655 5/1966 Bennett... 23/301 3,359,077 12/1967 Arst 23/301 3,453,352 7/1969 Goundry 264/93 OTHER REFERENCES Kirk & Othmer, Encyclopedia of Chemical Technology 2d, Vol.3, p. 679 (1964).

Primary Examiner-Yudkoff Assistant Examiner-R. T. Foster Attorneys-Samuel M. Mims, Jr., James 0. Dixon, Andrew M.

l-lassell, Harold Levine, Melvin Sharp and Richards, Harris & Hubbard ABSTRACT: An apparatus for pulling monocrystalline ribbons of silicon from a melt of silicon, which includes a shaping guide, through which the ribbon is pulled, having a heat retentive coating of a material, such as platinum, on the top surface of the guide. The shaping guide preferably includes 45 converging beveled lips on the bottom side thereof oriented toward the melt from which the ribbon is drawn for directing the melt into a rectangular slot formed in the shaping guide near the top surface. The rectangular slot serves to form the silicon in shape of a ribbon. The apparatus also preferably includes a heat reflective member supported above the shaping guide by a quartz pedestal. The heat reflective member serves 1 to assist the coating on the top surface of the shaping guide in maintaining a low temperature differential between the top and bottom surfaces of the shaping guide.

4! METAL WASHER PATENTEDHUVZ 19m 3.617.223

4'1 METAL WASHER 33 QUARTZ K PEDESTAL FIG. I

FIG. 2

HEAT- REFLECTIVE 2 v Mag comma 24 29 23 v K/ i; QUARTZ DISC 2 f 28 3 INVENTOR JAMES c. BOATMAN V. 5/74 M d ATTORNEY APPARATUS FOR FORMING MONOCRYSTALLINE RIBBONS F SILICON The invention relates to apparatus for producing crystalline semiconductor material, and more particularly to apparatus for producing mononcrystalline semiconductor material in ribbon form.

The increasing demand for greater reliability of semiconductor devices and continued pressures for reduced costs have produced extensive mechanization of semiconductor manufacturing processes. Such mechanization is based on the effective utilization of slices of single crystal material prepared from grown crystals and has provided certain improvements in quality and cost reduction. However, these techniques have been limited to batch-handling of large numbers of slices obtained from large rods of single crystal material. Furthermore, operations such as sawing, lapping polishing and etching of the crystal slices to prepare them for fabrication into semiconductor devices result in large wastes of the original monocrystalline semiconductor crystal. Only approximately 35 percent of the starting material is ultimately used in devices.

One approach suggested by the prior art for solving the above problem is directed to the production of semiconductor material in the form of a crystalline web from between dendrites. However, this process has many inherent limitations. Producing a silicon web between two dendrites requires that the dendrites be grown from a supercooled melt because dendritic growth actually occurs below the liquid level in supercooled regions. The dendritic growth temperature is about C. to about l5 C. below the normal melting point of the silicon. When this supercooled region is established, it must be held at essentially a fixed temperature by a control system which operates with a precision of about :0.0l C. Critical thermal gradients in the crucible and cover system used in the dendritic growth technique must be maintained to support the supercooled region necessary for dendritic growth; a complex and difficult control requirement for a manufacturing process.

The requirements for seed crystals which will initiate dendritic ribbon growth are quite critical. Suitable seeds must have twin planes, parallel to the growth direction and the number and spacing of these twin planes are critical. Furthermore, the twin planes and the seed are propagated throughout the growing web and are present in the finished material. The presence of dendrites along the edges of the semiconductor ribbon poses problems in device manufacturing'The use of this material for epitaxial depositions or diffusions generally requires the absence of the dendrites. Thus, the dendrites must be removed by scribing and breaking, etching or electron beam cutting. However, these techniques contaminate or damage the web material and add processing costs to the finished web.

The dendrite web growth requirement of the supercooled melt makes the formation of a long web impractical. The addition of new charge material to the melt would disturb the critical thermal gradients and terminate growth, thus limiting the total length of web which can be grown to the batch size.

Efforts have also been directed t the pulling of single crystal germanium ribbon through a slot in a graphite plate floating on the surface of molten germanium contained within a graphite crucible. Such technique, however, requires that the temperature in the region surrounding the graphite plate be controlled within :0.03 C., which is a difficult task, even h mpted unsle labqe s nd nawu This invention may be genera y descriFed as a crystaTgrowing apparatus of the type adapted to pull a ribbon shaped crystal of semiconductive material from a melt of the same upward through a shaping guide which comprises a quartz disc having an aperture therethrough and a heat retentive coating on the top surface of the disc for lowering the temperature differential between the top and bottom surfaces of the disc. The aperture through the disc preferably forms a rectangular opening in the bottom surface which is larger than the rectangular opening formed in the top surface so that beveled lips are formed at an angle of from 30 to 60 near the bottom of the disc. The lips terminate at a point intermediate the top and bottom surfaces of the disc so that the remainder of the aperture takes the form of a rectangular slot through which a ribbon may be drawn after the material has been directed into the rectangular slot by the beveled lips.

The apparatus may also include a heat reflective ring disposed above the top surface of the shaping guide to further lessen the temperature differential between the top and bottom surfaces of the shaping guide.

Other features and advantages of the present invention will become readily understood from the following detailed description taken in conjunction with the appended claims and attached drawing, in which:

FIG. 1 is a partially sectioned, side perspective view of one embodiment of the present invention;

FIG. 2 is an enlarged, exploded view of a shaping guide and associated structure, and

FIG. 3 is a cross-sectional view of the shaping guide illustrated in FIGS. 1 and 2.

With reference to FIG. 1, a cylindrical quartz chamber 11 is fitted with end plates 12 and 13. End plate 12 receives therethrough a top chuck 14, and end plate 13 receives therethrough a bottom chuck 15 both of which are associated with conventional ribbon pulling equipment. The bottom chuck 15 serves as a holding chuck for silicon feed rod 16 while top chuck 14 serves as a holding chuck for silicon seed l7. Disposed within chamber 11 between chucks 14 and 15 is a quartz ring 18 fused to the sidewall of chamber 11 to serve as a support platform for shaping guide 19, as illustrated in FIG. 1. Shaping guide 19, as more particularly illustrated in FIGS. 2 and 3, comprises a quartz disc 21 having an aperture therethrough, generally indicated by the reference numeral 22. Aperture 22 forms a rectangular opening 23 in the top surface 24 of disc 21 and a rectangular opening 25 in the bottom surface 26 of disc 21. The bottom opening 25 forms a wider rectangle than the top opening 23. Between the top opening 23 and the bottom opening 25 there are formed beveled lips 27 and 28 which terminate at a point below tip surface 24 to form a rectangular slot 29. The beveled lips 27 and 28 are formed at an angle of 45 relative to a line perpendicular to surface 24. The surfaces at each end of the aperture formed in disc 21 are vertical. Disc 21 is covered with a platinum coating 31, the purpose of which is explained hereafter. Quartz disc 21 is also provided, as particularly illustrated in FIG. 2, with a plurality of radial openings 32. Shaping guide 19 supports a quartz pedestal 33 which includes an inner cylindrical tube 34 and an outer cylindrical tube 35 which are joined in a concentric position by quartz spokes 36 and 40 at the top and bottom, respectively. The inner tube 34 and outer tube 35 are also provided with registering opening 37 and 38, respectively. The pedestal 33, which is supported on its bottom surface by shapin g guide 19, in turn supports upon its top surface a quartz ring 39. Quartz ring 39 supports a conventional metal washer 41 formed of iron or the like. The inner periphery 42 of washer 41 is somewhat elliptical in shape, as illustrated in FIG. 2.

A gas may be circulated through chamber 11 by way of inlet line 43 and exhaust 44, both of which communicate with the interior of chamber 11. Quartz chamber 11 is surrounded by an RF coil 45 for induction heating of silicon feed rod 16.

In operation, a silicon feed rod 16 is affixed to bottom chuck l5, and a single crystal seed of silicon is affixed to the top chuck 14. Rod 16 may be monocrystalline or polycrystalline, although the latter is less expensive and is ordinarily preferred for routine operations. End plates 12 and 13 are then affixed to chamber 11 in an airtight manner and the chamber 11 flushed with an oxygenfree inert gas, such as argon, for several minutes. The tip of feed rod 16 is then moved upward into proximity to the bottom surface 26 of shaping guide 19 and the radio frequency coil energized. The silicon feed rod 16, which has been preheated with a flame or an RF heater prior to insertion into chamber 11, will melt in the induction field created by Rf coil 45 to form a molten zone 46 which will be supported upon the end of rod 16 by its own surface tension. The rod is pushed upwardly against beveled lips 27 and 28 where it will initially freeze. Upon continued application of heat to rod 16 by RF coil 45, the silicon between lips 27 and 28 will melt and flow upwardly through slot 29. The seed 17 is then lowered to contact the molten silicon in slot 29, and upon contact, momentarily freezes. Within a short period under the influence of the RF heating, the tip of the seed will melt. The power output of RF coil 45 is then adjusted until the liquid-solid interface between the seed 17 and molten zone 46 is slightly above the shaping guide 19. Top chuck 14 is then gradually moved upward pulling a ribbon of single crystal silicon 47 from the melt 46. As the ribbon is pulled, the isothermal growth interface is maintained slightly above the surface 24 of shaping guide 19 to facilitate the growth of the ribbon 47. The ribbon 47 is formed as the molten silicon from the molten zone 46 is pulled upwardly over beveled lips 27 and 28 into rectangular slot 29. Since the silicon is maintained in a liquid state until it reaches a point about r-millimeter above surface 24, the rectangular slot 29 will serve to shape the molten silicon in the form of a ribbon, in which shape the silicon will solidify at a point immediately above slot 29 to fonn the single crystal silicon ribbon 47. As silicon is removed from molten zone 46, the bottom chuck 45 is moved upwardly at a rate sufficient to maintain a molten quantity of silicon in abutment with beveled lips 27 and 28. Molten zone 46 is maintained at a temperature between about 1,420 C., which is the melting point ofsilicon, and 1,500 C.

Platinum coating 31 serves to retain heat within shaping guide 19 for the purpose of minimizing temperature differential between the bottom surface 26 and the top surface 24 of shaping guide 19. in the absence of a heat retaining coating, the temperature of the top surface 24 of shaping guide 19 will be at a temperature of about 950 C. causing the ribbon 47 to freeze or solidify as it passes through shaping guide 19 resulting in polycrystalline growth or breakage of the ribbon 7. Use of a platinum coating 31 will increase the temperature of surface 24 about 200 C. permitting the pulling of a continuous ribbon 47.

The platinum coating 31 is formed by diluting a platinum paste, such as manufactured by Englehard, Inc. with a suitable solvent, such as kerosene, and applying multiple layers of the diluted paste to the surface 24 of quartz disc 21 in the configuration illustrated by FIGS. 1-3. Before applying a subsequent coat, the solvent is evaporated in a quartz tube which is heated with a hydrogen-oxygen torch. During heating, the kerosene evaporates depositing the platinum particles on the top surface of guide 19. Solid platinum will, under the influence of the heat, to which shaping guide 19 is subjected, melt, and is thus not preferred, An area 48 around top opening 23 is left uncoated in order to prevent contamination of silicon ribbon 46 by contact with the coating 31.

While the beveled lips 27 and 28 could extend upwardly to terminate in opening 23, thus eliminating the rectangular slot 29, such an arrangement is not preferred as the opening 23 will become quickly fire-polished allowing the molten silicon to roll over upon the surface 24. Further, beveled lips 27 and 28 could be eliminated and the rectangular slot 29 extended between the top surface 24 and bottom surface 26, but the vertical temperature gradient across the shaping guide 19 would become very difficult to control as the molten silicon would solidify within the slot 29. Thus, it is preferred that the depth of slot 29 be greater than about mils and less than about mils for best results. it is also preferred that the angle of the bevel of lips 27 and 28 be greater than about 30 and less than 60. Specifically, if the included angle of the bevel is greater than 60, relative to a line perpendicular to the shaping guide 19, the temperature gradient across the shaping guide 19 is not great enough, causing fire-polishing of the slot 29. Once slot 29 is fire-polished, dimensional control of ribbon 47 is lost. if the included angle of the bevel is less than 30, the ribbon 47 will tend to freeze out within shaping guide 19 as the temperature differential between top surface and the bottom surface 26 will be too great.

While the use of the shaping guide 19, illustrated in FIG. 3, may be used to produce single crystal silicon ribbon, better control of the isothermal interface and thus more uniform ribbons can be produced by inclusion of the heat reflecting ring or washer 41. The washer 41 will serve to reflect heat emitted from the top surface of shaping guide 19 back onto the shaping guide 19 as well as undergo some induction loading and radiate heat onto shaping guide 19. Metal washer 41 is maintained a sufficient distance from shaping guide 19 by quartz pedestal 33 to prevent melting of washer 41 by the RF coil 45, and at the same time is sufficiently close to reflect enough heat onto the top surface of shaping guide 19 to maintain the temperature of the top surface at about l,350 C. During the pulling of the ribbon 47 from the molten zone 46, a quantity of inert gas, such as argon is continuously flushed through chamber 11 to remove any impurities which might enter around gas seals 20. The argon flows through inlet line 43 upward through chamber 11, through radial openings 32 in shaping guide 19, between the inner wall 34 and outer wall 35 of pedestal 33 and around quartz washer 39 and metal washer 41 before it exits through discharge line 44.

The quartz pedestal 33 is preferably sandblasted to further assist in minimizing heat radiation from shaping guide 19, and is provided with the openings 37 and 38 to permit a viewing of the ribbon 47 by an operator to insure that the ribbon 47 is solidifying at a point above rectangular slot 29.

While the preferred coating for shaping guide 19 is platinum, applied in a laminate form, as described above, other materials such as tungsten, palladium, molybdenum, tantalum and niobium may be also employed, as all of these metals will form a heat retentive deposited coating on the surface of the quartz disc. As with the platinum, these materials are preferably applied in the paste form, and in a multilayer manner.

While rather specific terms have been used in describing several embodiments ofthe present invention, they are not intended, nor should they be construed as a limitation upon the invention as defined by the following claims.

What is claimed is:

1. in a crystal growing apparatus of the type adapted to pull a ribbon-shaped crystal of silicon from a melt of the same upward through shaping guide, an improved shaping guide which comprises:

a quartz disc having an aperture therethrough and a heat retentive coating on the top surface of said disc for lowering the temperature differential between the top and the bottom surfaces of said disc, wherein said coating is deposited from a suspension of a metal selected from the group consisting of platinum, tungsten, palladium, molybdenum, tantalum and niobium. 2. In a crystal growing apparatus of the type adapted to pull a ribbon-shaped crystal of silicon from a melt of the same upward through a shaping guide, an improved shaping guide which comprises:

a quartz disc having an aperture therethrough; and a heat retentive coating on the top surface of said disc for lowering the temperature differential between the top and the bottom surfaces of said disc, wherein said coating is a metal selected from the group consisting of platinum, tungsten, palladium, molybdenum, tantalum and niobium. 3. The apparatus of claim 2 wherein said coating is platinum.

4. The apparatus of claim 2 wherein said coating is tungsten. 5. The apparatus of claim 2 wherein said coating is palladium.

6. The apparatus of claim 2 wherein said coating is molybdenum.

7. The apparatus of claim 2 wherein said coating is tantalum.

8. The apparatus of claim 2 wherein said coating is niobium.

9. In a crystal growing apparatus of the type adapted to pull a ribbon-shaped crystal of silicon from a melt of the same upward through a shaping guide, and improved shaping guide which comprises:

a quartz disc having an aperture therethrough; and

a heat retentive coating on the top surface of said disc for lowering the temperature differential between the top and the bottom surfaces of said disc, wherein said coating is patterned to provide an uncoated area around the aperture in the top surface of said disc.

10. In a crystal growing apparatus of the type adapted to pull a ribbon-shaped crystal from a melt of the same upward through a shaping guide, an improved shaping guide which comprises:

a quartz disc having an aperture therethrough; and

a heat retentive coating on the top surface of said disc for lowering the temperature differential between the top and the bottom surfaces of said disc;

a heat retentive ring positioned above said shaping guide for reflecting heat onto the top surface of said shaping guide; and

means for supporting the heat reflective ring above the shaping guide comprising a quartz pedestal formed of an inner cylindrical tube,'the bottom of which rest upon the top surface of the shaping guide and an outer cylindrical tube concentrically positioned around the inner cylindrical tube with its bottom surface supported upon the top surface of the shaping guide.

11. The apparatus of claim 10 which includes a quartz ring interposed between the top surfaces of said inner and outer cylindrical tubes and said heat reflective ring.

12. The apparatus of claim 11 wherein the inner and outer cylindrical tubes are provided with registering ports throughout the bottom portions thereof to permit viewing of the top opening in the shaping guide.

13. The apparatus of claim 1 wherein said coating comprises plural layers of a metal deposited from a suspension, the metal being selected from the class consisting of platinum, tungsten, palladium, molybdenum, tantalum and niobium.

14. The apparatus of claim 13 wherein said metal is platinum.

15. The apparatus of claim 13 wherein said metal is tungsten.

16. The apparatus of claim 13 wherein said metal is palladium.

17. The apparatus of claim 13 wherein said metal is molybdenum.

18. The apparatus of claim 13 wherein said metal is tantalum.

19. The apparatus of claim 13 wherein said metal is niobium.

i i i 

2. In a crystal growing apparatus of the type adapted to pull a ribbon-shaped crystal of silicon from a melt of the same upward through a shaping guide, an improved shaping guide which comprises: a quartz disc having an aperture therethrough; and a heat retentive coating on the top surface of said disc for lowering the temperature differential between the top and the bottom surfaces of said disc, wherein said coating is a metal selected from the group consisting of platinum, tungsten, palladium, molybdenum, tantalum and niobium.
 3. The apparatus of claim 2 wherein said coating is platinum.
 4. The apparatus of claim 2 wherein said coating is tungsten.
 5. The apparatus of claim 2 wherein said coating is palladium.
 6. The apparatus of claim 2 wherein said coating is molybdenum.
 7. The apparatus of claim 2 wherein said coating is tantalum.
 8. The apparatus of claim 2 wherein said coating is niobium.
 9. In a crystal growing apparatus of the type adapted to pull a ribbon-shaped crystal of silicon from a melt of the same upward through a shaping guide, and improved shaping guide which comprises: a quartz disc having an aperture therethrough; and a heat retentive coating on the top surface of said disc for lowering the temperature differential between the top and the bottom surfaces of said disc, wherein said coating is patterned to provide an uncoated area around the aperture in the top surface of said disc.
 10. In a crystal growing apparatus of the type adapted to pull a ribbon-shaped crystal from a melt of the same upward through a shaping guide, an improved shaping guide which comprises: a quartz disc having an aperture therethrough; and a heat retentive coating on the top surface of said disc for lowering the temperature differential between the top and the bottom surfaces of said disc; a heat retentive ring positioned above said shaping guide for reflecting heat onto the top surface of said shaping guide; and means for supporting the heat reflective ring above the shaping guide comprising a quartz pedestal formed of an inner cylindrical tube, the bottom of which rest upon the top surface of the shaping guide and an outer cylindrical tube concentrically positioned around the inner cylindrical tube with its bottom surface supported upon the top surface oF the shaping guide.
 11. The apparatus of claim 10 which includes a quartz ring interposed between the top surfaces of said inner and outer cylindrical tubes and said heat reflective ring.
 12. The apparatus of claim 11 wherein the inner and outer cylindrical tubes are provided with registering ports throughout the bottom portions thereof to permit viewing of the top opening in the shaping guide.
 13. The apparatus of claim 1 wherein said coating comprises plural layers of a metal deposited from a suspension, the metal being selected from the class consisting of platinum, tungsten, palladium, molybdenum, tantalum and niobium.
 14. The apparatus of claim 13 wherein said metal is platinum.
 15. The apparatus of claim 13 wherein said metal is tungsten.
 16. The apparatus of claim 13 wherein said metal is palladium.
 17. The apparatus of claim 13 wherein said metal is molybdenum.
 18. The apparatus of claim 13 wherein said metal is tantalum.
 19. The apparatus of claim 13 wherein said metal is niobium. 